This invention relates to fuel cell production systems and, in particular, to a fuel cell system integrated with a heat engine such as an internal combustion engine and an external combustion engine.
A fuel cell is a device, which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode electrode and a cathode electrode separated by an electrolyte, which serves to conduct electrically charged ions.
A fuel cell may be combined with a heat engine such as a turbine generator to produce a high efficiency system, commonly called a hybrid system. In a commonly practiced hybrid system, a fuel cell typically is situated in the position normally occupied by the combustor of the turbine generator so that air compressed by the turbine generator compressor section is heated and then sent to the fuel cell cathode section. In this arrangement, the fuel cell is operated at a high pressure, which substantially increases the cost of the power plant hardware. The high pressure operation of the fuel cell inhibits the use of internal reforming in the fuel cell which further increases the plant cost and reduces efficiency, and subjects the fuel cell to potentially damaging pressure differentials in the event of plant upset. Furthermore, the fuel cell pressure is coupled with the gas turbine pressure, reducing the reliability of operation and limiting the application to system designs where the gas turbine pressure is nearly matched with the fuel cell pressure.
To overcome these disadvantages, another hybrid system has been developed where a heat engine in the form of a turbine generator is bottomed with a fuel cell so that the heated air discharged from the gas turbine is delivered to the cathode section of the fuel cell. U.S. Pat. No. 6,365,290, assigned to the same assignee hereof, discloses such a hybrid fuel cell/gas turbine system, in which waste heat from the fuel cell is used by a heat recovery unit to operate the heat engine cycle, and the system is arranged such that the compressed oxidant gas, heated in the heat recovery unit and by a high temperature heat exchanger, is expanded in the expansion cycle of the heat engine. It is then passed through an oxidizer (also commonly called combustor or burner), which also receives the anode exhaust, passed through the heat exchanger and the resultant gas delivered to the cathode section of the fuel cell.
In a modification of the system of the '290 patent, U.S. Pat. No. 6,896,988 assigned to the same assignee hereof, discloses an enhanced hybrid system for high temperature fuel cells including solid oxide and molten carbonate fuel cells which expands the applicability of the system of the '290 patent by providing a better match between the fuel cell size and the gas turbine. Despite the benefits provided by the system of the '988 patent, there are limitations to its application.
Both the systems of the '290 and '988 patents are intended for generation of electricity at very high efficiencies. However, they require a relatively high temperature heat exchanger, which may be very costly especially for large-scale power plants (>10 MW). Also, the aforesaid systems need to be more flexible if they are to be used in system configurations which are needed to better satisfy compromises between efficiency and power generation.
It therefore would be desirable to provide an alternative fuel cell and heat engine hybrid system having high efficiencies and providing greater flexibility in balancing power between a high temperature fuel cell such as a solid oxide or molten carbonate fuel cell and a heat engine assembly such as a compressor/turbine assembly.
It would also be desirable to provide a fuel cell and heat engine hybrid system which is better able to mitigate against the need for a high temperature heat exchanger (recuperator or regenerator) to be operated above the fuel cell cathode exhaust temperature.
It would further be desirable to provide a hybrid fuel cell and heat engine hybrid system, which may offer a less expensive alternative system configuration to the ones described in '290 and '988 patents for high temperature and near atmospheric pressure fuel cells.